1. Field of the Invention
The present invention relates to a turbo-molecular pump used, for example, for evacuation in a vacuum chamber and a method of assembling the turbo-molecular pump.
2. Description of the Related Art
Equipment using a vacuum device which accomplishes evacuation by using a vacuum pump and the interior of which is kept in vacuum includes semiconductor manufacturing equipment, liquid crystal manufacturing equipment, electron microscopes, surface analyzers, microfabrication equipment, and the like.
Also, among various types of vacuum pumps, a turbo-molecular pump is often used to realize a high-vacuum environment.
The turbo-molecular pump is configured so that a rotor rotates at a high speed in a casing having an intake port and an exhaust port. On the inner peripheral surface of the casing, stator blades are disposed in multiple stages, and on the other hand, on the rotor, rotor blades are disposed radially in multiple stages.
When the rotor rotates at a high speed, gas is sucked through the intake port and discharged through the exhaust port by the action of the rotor blades and stator blades.
The aforementioned rotor has a substantially cylindrical shape one end of which is closed, and at the end on the closed side, a rotor shaft (rotating shaft) is fixed. The rotor blades are formed in multiple stages from the intake port side toward the exhaust port side (from the upstream side toward the downstream side) so as to project radially from the outer peripheral wall surface of the rotor.
The rotor shaft of the turbo-molecular pump rotates at a high speed close to the motion velocity of gas molecule, so that a high centrifugal stress acts on the rotor blades due to this rotation. The centrifugal force acting on the rotor blades increases toward the lower stage (downstream side).
Thereupon, a technique for restraining breakage by relaxing the centrifugal stress has conventionally been proposed in the following Patent Document.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-246197
Patent Document 1 proposes a turbo-molecular pump having a construction such that for the rotor blades provided in multiple stages, the outside diameters of the rotor blades on the exhaust port side are smaller than the outside diameters of the rotor blades on the intake port side.
By using such a construction, the centrifugal stress acting on the rotor blade and the support part thereof on the downstream side (the exhaust port side) when the rotor rotates at a high speed can be reduced, and therefore the exhaust properties of pump can be improved while restraining local stress and temperature rise.
However, the above-described turbo-molecular pump having a construction such that the outside diameters of the rotor blades on the exhaust port side are smaller than the outside diameters of the rotor blades on the intake port side as described in Patent Document 1 has a problem in that a method of assembling stator blades and spacer rings is restricted as compared with a turbo-molecular pump in which the outside diameters of rotor blades in all stages are equal.
The spacer ring is a positioning member for providing a necessary clearance between the stator blades.
For example, a case where the spacer ring is formed integrally, that is, formed into a ring shape continuous in the circumferential direction is explained.
To restrain the reduction in exhaust properties, the turbo-molecular pump has a construction such that a clearance between the inner wall of spacer ring and the outside diameter of rotor blade is decreased to prevent the backflow of gas.
Therefore, the stator blades cannot be piled up one after another from the downside (from the exhaust port side) while the spacer rings are fitted from the intake port side of rotor blade because the rotor blade on the intake port side and the spacer ring on the exhaust port side interfere with each other.
Conventionally, a method has been used in which the spacer rings are halved like the stator blades, and the stator blades are piled up one after another from the downside (from the exhaust port side) while being inserted from the radial direction.
However, for such a halved spacer ring, at the time of fabrication, that is, at the time of cutting, the cut surface may be deformed, or the external shape may be distorted.
Also, for the turbo-molecular pump using the halved spacer rings, the strength against breaking torque at the time of abnormality decreases as compared with the turbo-molecular pump using integral spacer rings that are not halved.